Nocardia, an organism closely related to Mycobacterium, can cause fatal infections in man. Little is known about the mechanisms controlling nocardial invasion, infection, disease production and host immunity. In contrast, a great deal of work has been done on mycobacterial diseases. It has been shown that host immunity to mycobacteria is mainly a cell-mediated type. Further, it has been shown with mycobacteria that much of the organism's effect on the host and host immunity as well as the organism's virulence resides in the chemical make-up of the cell envelope. Therefore, I propose to study the in vivo and in vitro chemical composition of nocardial cells. I shall explore the effect of the host as well as the environment on these components in order to determine how they affect the host-parasite relationship and host immunity.